This invention relates to processes and compositions for separating suspended solids from aqueous suspensions containing them.
Aqueous suspensions are usually clarified by the addition of one or more water soluble organic polymers that are commonly referred to in the industry, and are described herein, by the generic term "flocculant". In fact, these "flocculants" can be of two entirely different types that operate by entirely different mechanisms. The true flocculant (herein referred to as a "bridging flocculant") is a high molecular weight polymer that achieves its flocculation effect primarily by a bridging mechanism. Bridging flocculants must have high molecular weight, generally above 5 million and frequently above 10 million. When expressed in terms of intrinsic viscosity, this is usually at least 5 dl/g and often above 10 dl/g.
Molecular weights mentioned herein are measured by gel permeation chromotagraphy and intrinsic viscosities are measured by suspended level viscometer at 25.degree. C. in 1 molar aqueous sodium chloride buffered to pH 7.0.
The other type of "flocculant" is the material that is more accurately referred to as a "coagulant". It is a lower molecular weight, highly ionic, material that achieves its flocculation effect (i.e., coagulation) primarily by adsorbing on to the solid particles in the suspension and changing the surface charge on them, with little or no bridging effect between solid particles. These coagulants must have high ionic charge and typically at least 50% of the monomers, frequently at least 80%, by weight of the monomers from which they are formed carry ionic charge. They must have low molecular weight and typically this is below about 1.5 million and, when they are anionic, the molecular weight is often very much less, typically below 500,000. Expressed in terms of intrinsic viscosity, the coagulants generally have IV below 3 dl/g, often below 2 dl/g.
There are many instances where it is desirable to flocculate a suspension of suspended solids by addition of two flocculants that are incompatible with each other in solution, in the sense that they tend to interact physically or chemically with each other rather than with the suspended particles. For instance when equal volumes of 0.5% aqueous solutions of the two polymers are combined they may give a precipitate or a gel.
A common example of this incompatibility arises with highly counterionic flocculants. The most effective anionic flocculants are generally present as alkali metal salts, and the most effective cationic flocculants are generally present as quaternary ammonium or acid addition salts of tertiary amine polymers. Although, for instance, the corresponding free acid polymer may be compatible with, for instance, the free base or acid addition salt polymer, the alkali metal salt version of the anionic polymer tends to be highly incompatible with the acid addition or quaternary ammonium salt polymers. This is unfortunate, as these combinations tend to be the most effective flocculants.
Accordingly it is necessary, at present, to keep them apart and if both types are to be used they are both generally dosed sequentially into the suspension as preformed aqueous solutions. In a typical process, a coagulant is added to cause coagulation-flocculation and a bridging flocculant is added subsequently to bridge-flocculate the coagulated material.
Because the coagulant polymers are of low molecular weight and high ionic content, they have low solution viscosities and so normally are provided as aqueous concentrates. These can be dosed direct into the suspension or they can be diluted in-line prior to dosing. The supply of aqueous concentrates involves the transport and packaging of large amounts of water and it might have been thought to have been desirable to supply the coagulant polymers in solid form instead. Methods of making water soluble polymers in, for instance, bead form are well known. However, coagulant polymers have to be in solution before they can act as coagulants and so normal considerations would dictate that the user should provide make-up apparatus for dissolving the solid coagulant polymer into water prior to dosing the resultant solution into the suspension. Accordingly, there has been little incentive for the user to add this make-up apparatus in preference to buying the normal aqueous concentrate and dosing it direct, or diluting it in-line.
Accordingly, the great majority of coagulant polymers are still provided as aqueous concentrates. For instance, a widely used coagulant polymer is polydiallyldimethyl ammonium chloride. Many manufacturers supply this as aqueous concentrate but there is only one solid grade of it, supplied by Allied Colloids Inc. and Allied Colloids Ltd. under the trade names Magnafloc 368 and Percol 368.
Bridging flocculants have much molecular weight and so cannot conveniently be supplied as aqueous concentrates and, instead, have to be supplied as dispersions in oil or as powders. It is well known that many of them dissolve rather slowly and so in practice all conventional systems for adding bridging flocculants to a suspension involve make-up apparatus for mixing the powder or dispersion with water and for holding it for a prolonged period, generally at least an hour, to allow dissolution to go to substantial completion.
Although normal practice therefore requires that coagulant polymers should be added as aqueous concentrates or diluted aqueous concentrates and that bridging flocculants should be made up into an aqueous solution over a period of at least an hour before dosage, there have been some instances where solid bridging flocculants have been added direct to a suspension. For instance it is known to add blocks or particles of flocculant to a flowing suspension where relatively crude separation is sufficient, and in some of these systems inorganic coagulant is included as well.
Because of the problems of incompatibility between similar amounts of highly counterionic polymers, all normal processes for using similar amounts of counterionic polymers therefore always involve sequential dosing of preformed solutions. For instance JP 63218246 describes various polymer blends and shows that an insoluble product is obtained when there is dissolved into water a blend of powdered acrylamide-sodium acrylate copolymer and powdered acrylamide-quaternised dimethylaminoethyl methacrylate copolymer.
In JP-A-58215454 powder blends are formed of an acrylamide acrylic acid copolymer and a diethylaminoethyl methacrylate polymer. However the cationic polymer is in the form of the free base, the anionic polymer is in the form of the free acid, and the blend includes another acid, amido sulphonic acid, presumably to ensure that the anionic polymer is in free acid form so as to maintain compatibility.
In JP-A-60202787 there is a description of demulsifying an aqueous oil emulsion using various polymers and polymer blends, but the processes are described as requiring air agitation for an hour or more and clearly these are not relevant to the separation of suspended solids since coagulation occurs very rapidly.